Concept:A tissue is a group of similar cells that work together to carry out a specific function.Explanation:Option A: A vessel element is a single cell found in xylem tissue.Option C: A sieve tube element is a single cell found in phloem tissue.Option D: An erythrocyte is just a red blood cell.Option B: Blood is made up of red blood cells, white blood cells, platelets, and plasma.These components work together, so blood is classified as a connective tissue.Answer:B. Blood